Effect of solvent polarity on the assembly behavior of PVP coated rhodium nanoparticles

The rhodium nanoparticles (RhNPs) were prepared in aqueous, ethanol, and water–ethanol mixed solution using rhodium chloride, sodium borohydride and polyvinylpyrrolidone (PVP) as precursor, reducing agent and stabilizer, respectively. The effect of solvent polarity on the assembly behavior of PVP coated RhNPs was systematically investigated by varying the water-to-ethanol ratio in the solution. The main product changed from small separated RhNPs in ethanol to large spherical assemblies of RhNPs in water. The strong tendency for the RhNPs to form aggregates in high polarity solvent was clearly demonstrated by increasing the water-to-ethanol ratio in the reaction media. TEM characterization revealed a two-step formation process of the spherical RhNP assemblies in water: the initial rapid particle generation step followed by a slow particle aggregation step. A mechanism was proposed to elucidate the aggregation behavior of PVP coated RhNPs. The driving force for the aggregation of RhNPs is ascribed to the hydrophobic interaction of hydrocarbon chains of PVP, which can greatly reduce the surface energy of PVP coated RhNPs in high polarity solvents. This work shed light on how to control the formation of nanoparticle aggregates by modulating the polarity of solvent.